During the production process of a bottom gate type thin film transistor, the etching process of an active layer will typically result in a certain degree of damage to the surface of a gate electrode protection layer (a gate insulation layer) so that the surface of the gate electrode protection layer will become rough and uneven, thereby resulting in coarse crystal grains present in the source-drain metal layer later deposited on the gate electrode protection layer. For the following production processes, if the source-drain metal layer contains coarse crystal grains, then the surface of the source-drain metal layer will become rough, which may lead to problems such as abnormal morphology of the connection portion of the source-drain metal layer in a connection via hole, the disconnection of ITO deposited in the connection via hole, and the susceptibility of corrosion of the source-drain metal layer that is disconnected, which has an adverse influence on the product yield of the display panel.